1. Field of the Invention
This invention relates generally to optical systems and is concerned more particularly with an optical system including a plate-like beamsplitter having an optically symmetrical structure.
2. Discussion of the Prior Art
An optical beamsplitter may comprise an optical plate made of transparent material and serving as a substrate for supporting on one of its extended flat surfaces a semi-transparent film of light reflective material. When used for splitting a light beam or light image, the beamsplitter is disposed interceptingly in the path of the beam or image and oriented at an angle to said path. As a result, a fractional component of the light beam or image will be transmitted through the film and continue to travel in the original direction of propagation. Also, a fractional component of the light beam or image will be reflected from the angularly oriented film of the beamsplitter and travel in a direction extending at an angle to the original direction of propagation.
When used for combining two light beams or light images travelling along intersecting paths, the beamsplitter is disposed at the intersection of the two paths and oriented at an angle to each of said paths. As a result, one of the light beams or images will be transmitted through the film of the beamsplitter and continue to travel in the original direction of its propagation. The other light beam or image will be reflected from the angularly oriented film of the beamsplitter and travel in the same direction as the light beam or image transmitted through the film. Consequently, if the beamsplitter film constitutes a plane of symmetry between the two angularly disposed sources, the transmitted image and the reflected image travelling from the beamsplitter to the eye of an observer will appear to be superimposed on one another and form a composite image.
Accordingly, there is disclosed in the prior art a number of optical systems, such as used in photographic rangefinders and measuring microscopes, for examples, wherein beamsplitters are disposed for superimposing an image of a reticle or the like on an image of an object being viewed. Also, the prior art includes a number of multi-color image projection systems which use beamsplitters for superimposing different color images of the same object on one another to produce a multi-color composite image. For example, in a copending application Ser. No. 350,469 filed on Feb. 19, 1982 by the present applicant and assigned to the present assignee, there is disclosed "Multi-Color Image Display Apparatus" comprising an optical system coupled to a planar output screen of a display tube for combining two mirror images of the same object produced in different colors, such as red and green, for examples, on respective half-portions of the output screen.
The optical system described in the referenced application Ser. No. 350,469 includes a conventional plate-type beamsplitter aligned with the green light producing half-portion of the output screen and oriented at an angle with respect to the plane of the screen. This beamsplitter comprises a transparent optical plate supporting a film of material which is partly transmissive and partly reflective to green light. Consequently, the green light image emanating from the aligned half-portion of the output screen has a fractional component transmitted through the film in the direction of its original propagation and a fractional component reflected from the film in an orthogonal direction relative to the original direction of propagation. The reflected component is reflected back from a perpendicularly disposed first mirror to the beamsplitter where it is transmitted through the beamsplitter film to an eye of an observer.
In order to recover the beamsplitter transmitted component of the green light image, there may be disposed beyond the beamsplitter and parallel with the output screen a second mirror which is orthogonal to the first mirror. The second mirror reflects the beamsplitter transmitted component of the green light image back to the film of the beamsplitter where it is reflected in the same direction as the beamsplitter reflected component of the green light image leaving the beamsplitter and travelling toward the eye of the observer. As a result, the original beamsplitter transmitted and reflected components of the green light image will appear to the eye of the observer to be superimposed on one another and form a composite green light image, provided that the direction of observation is close to parallel with the optical axis.
However, it has been found that if the direction of observation is changed to be appreciably non-parallel to the optical axis, there may be a noticeable separation of the beamsplitter transmitted and reflected components of the green light image which causes the appearance of a double green image at the eye of the observer. For a given angle of view, the optimum location of the second mirror is dependent upon the refractive index and thickness of the beamsplitter substrate plate, as well as upon which surface of the plate is supporting the film. Unfortunately, the optimum location of the second mirror also is a function of the angle of view.
To correct this problem, it may be proposed that the beamsplitter be of the prism type comprising two right-angle prisms having the semi-transparent film of light reflective material sandwiched between the hypoteneuse surfaces of the two prisms. Also, the first and second mirrors may comprise silvered outer surfaces of the two prisms. However, for projection of large images, the prisms would be uncommonly large and, therefore, excessively heavy. Furthermore, it is difficult and costly to achieve adequate optical quality for image projection in large prisms. Moreover, the resulting bulkiness of the large prisms would require a cumbersome mechanism for moving or rotating the beamsplitter off the optical axis in a system which requires such a maneuver.